Hardening of polyvinyl alcohol coatings on paper



Patented Oct. 12, 1954 HARDENIN G IOF-PIOLYVINYL ALCOHOL COATINGS ON PAPER William J. Priest, Rochester} N; Y-., assignor to EastmanrKodak Com pany, Rochester, N. 'Y., a

corporation of NewfJersey No Drawing. Application August5, 1952, Serial No. 302,825;

8 Claims, (01. 117-60) This invention comprises a process; of: coating on paper of compositions containing water-soluble hydroxyl-containing polymers, and for renderingthese coatings water-insoluble when they have been dried and cured;

In the past, trouble hasbeen experienced in the processing of some photographic papers due to the fact that paper ordinarily is not adapted for the soaking operations which are used in developing, fixing and washing the photographic emulsion. Not only has the paper support portion of the photographic paper been susceptible to change, such as loss ofwet strength, but also the photographicemulsion itself has been adversely.

affected, as'evidenced by blistering, frilling and the like.

Previous attempts have been madeto increase the wet strength of paper, but in many of those previous cases it has also been necessary'to incorporate plasticizer in the paper due to the brittleness tendencies induced in thepaperin increasing its wet strength because of either the nature of the material incorporated in the paperqor the amount of the material which was necessary. In

the case-0f many paperstreatedto improve theirwetstrength, the sensitized emulsioncoated. thereon hasbeen deleteriously afiected. by the.

wet strength imparting material used- Recently, the use of polyvinyl. alcohol or. resins having a large proportion of polyvinyl alcohol, has

been suggested for photographic purposes, such as for instance, a carrier for a light-sensitive material. For many purposes, however, polyvinyl alcohol or the polyvinyl acetates, which are watersoluble by virtue of their polyvinyl alcohol content, have not been regarded as sufiiciently resist-r ant to they effects of water'or of elevated temperatures. Consequently; water resistant polyvinyl compounds have been developed containing water-soluble reaction products of formaldehyde.

and melamine which, when prepared under desirable conditions, cause polyvinyl coatings to harden.

sufliciently so that processing, ferro-typing, and the like-can be carried out without detrimentally.

affecting either the physical or photographic properties of the product so treated. Aqueous coating compositions containing methylolmela mine; polyvinyl alcohol, and low molecular weight condensation products thereof, gel when the pH is adjusted to about 4. The hardening rate also is a function of the pH, requiring a longer time to harden as the pH approaches 7. However, the viscosity increase of these compositions upon standing renders the handling of these materials rather critical and limits the coating time and total quantity of coating composition which can be handled in one batch; Therefore, even in small batches, itis necessary continually to compensatejby coating machine adjustments, for increases in viscosity of such materials.

I'have found that in systems forrendering polyvinyl alcohol compositions insoluble in water at elevated temperatures by use of anaemia catalyst which come within the purview of this invention, if the catalyst is kept apart from the coating composition until after the coating containing the polyvinyl alcohol and hardening'addenda' has been deposited on the porous matrix, the viscosity increase is deferred until the coating mixes with the catalyst. This renders such coatingmaterials immensely more useful and is accomplished byincorporating the acidic catalyst into or onto the matrix prior to application of the coating containing the polyvinyl alcohol plus the hardening or insolubilizing additive. On application of thelatter coating some of the catalyst in the substratum di-fiusesinto the coating composition in the period in Which the latter is still fluid; and the process of "hardening or insolubilization is actuated. The

amount of catalyst and-consequent time and degree of hardening or insolubliziation may be regulated by the amount of catalyst which is incorporatde on or in the substratum.

Theinventionis particularly applicable to coating absorbent surfaces such as paper and the like, but is also applicable to the coating of other sur faces of aless absorbent nature.

One object of my invention is to provide a new method of hardening a coating composition,.par-- ticularly those containing-polyvinyl alcohol and-a:

hardening agent; comprising pretreating the surfaceto be coated with asuitable catalyst'before applying the coating. Another object of my invention is to provide acoating material as referred to; above which is readilymaintainediat substan-.

tially constant viscosity, thus eliminating the inconvenience occasioned by undesirablev viscosity increases during the coating operation. Afurther object of my invention is to provide methods for coating water soluble polyvinyl alcohol compositions on paper or other porous surfaces, which compositions'under mild conditions of drying and curing became substantially water insoluble. Another object of my invention is to provides. coating for photographic paper imparting good wet strength to the paperwithout brittleness or the necessity of plasticization; A further object of my invention'is to provide a coating for photographic'paper in which substantially no A. METHYLOL-MELAMINE 150 :parts 20 formaldehyde and 30 parts melamine were mixed and adjusted to a pH of '7. This mixture was stirred for two hours at 70-75 (2., during which time the melamine, dissolved, and later a small quantity of sediment separated. The clear supernatant liquid was removed by filtration at the end of the heating period.

B. POLYVINYL ALCOHOL 150 parts af aqueous polyvinyl alcohol gel, containing 17% polyvinyl alcohol, which was made by substantially complete deacetylation of a V-22 polyvinyl acetate (for the latter polymer, a solution containing 8.6 grams of polyvinyl acetate per 100 cc. of benzene has a viscosity of 22 op.) were dissolved in 330 parts water by heating at 85 C. for thirty minutes. To this solution were added 6.5 parts sodium nitrate dissolved in 20 parts of water. i

To 456.5 parts of mixture B were added 25 parts of mixture A. This was divided into three equal portions which were treated as follows:

Portion 1: adjusted to pH 4 with citric acid after having added .3 part of sodium acetate,

Portion 2: adjusted to pH 5.5 with citric acid after having added .3 part of sodium acetate.

Portion 3: adjusted to pH '7 and 10 parts of a phosphate buffer of the Clark and Lubs type were added.

Portion 1 rapidly became turbid on standing at room temperature. The mixture set to a gel within two hours. Viscosity measurements were conducted on portions 2 and 3 at 25 C. with the following results:

Table I Viscosity (centipoises) Time after mixing (hours) Portion 2 Portion 3 From the foregoing data it is evident that interaction in the methylol-melamine-polyvinyl alcohol system is strongly pH dependent.

The following examples illustrate the principle involved in my invention:

EXAMPLE 1 .Coatz'ng composition (C).To 25 parts of a methylol-melamine condensate made by heating 30 parts of melamine with 150 parts 20% aqueous formaldehyde at pH 7 for two hours at TO- C. were added 456.5 parts of an aqueous solution containing 1.35% of sodium nitrate and 5.3% of polyvinyl alcohol, the viscosity of which was 22 centipoises/molar solution in water. Seventy-five parts of this composition were adjusted to .pH of 7 and buiiered at this [pH with a phosphate buffer. To this were added 1.7 parts of potassium bromide dissolved in a little water and finally, with stirring, 2.1 parts of silver nitrate in water.

Following the dip coating, the papers were immediately hung in a vertical position and allowed to dry at room temperature. After drying and curing for various periods, at room temperature, samples of the papers were developed in Kodak D-72 developer (Langes Handbook of Chemistry, fifth edition), washed in dilute acetic acid and held in running water at about 70 C. for one minute. They were then removed and rubbed for assessment of the resistance of the coating to disintegration. The following table shows the results obtained:

Table I I Percent Curing qa 1e Citric Time at Properties of Hot Waterp Acid in Room Temsoftened Coating Sub Coat perature Days 10 1 Hard, no wash or rub ofi of pigment. 5 1 Hard; Pigment rubs ofi with abrasion. 2% l Partially hard; Pigment rubs 05. 0 1 Pigment washes ed. 10 2 Same as A above.

5 2 Do. 2% 2 Do. 0 2 Pigment washes ofi.

EXAMPLE 2 Portions of baryta-coated papers wer dip coated in solutions of hydrated aluminum sulfate (A12(S O4) 3.181120) varying in concentration from 0 to 10%. These were air dried at room temperature and then coated with coating composition C shown in Example 1. Procedures for coating, drying, curing and evaluation were the same as those used in Example 1.- Results of the evaluations are shown in Table III:

Table III Curing Percent Time at Properties of Hot Sample A1z (BO4)3.18HIO Room Water-softened in sub coat Tempera- Coating ture Days

.A. l0 1 Hard, but pigment rubs ofi with vigorous rubbing.

B 5 1 Pigment rubs off with moderate abrasion.

O 2% 1 Washes ofi.

D 0 1 Do.

E 10 2 Hard and resistant to abrasion.

F 5 2 Hard and good abrasion resistance.

G 2% 2 Partially hard,

abraids easily.

H 0 2 Washes ofi.

EXAMPLE 3 Portions of baryta-coated paper, were dip coated in solutions of hydrated aluminum sulfate A12(SO4)3.18H2O varying in concentraton from 10 to percent, then air dried at room temperature. One set of papers was given two dipcoatings in the 10% solution. The papers so treated were dip coated in composition (D), made as follows:

154 parts of a 6.5% aqueous polyvinyl alcohol, viscosity 22 centipoises/molar solution in water .5 part ammonium chloride in parts water 4.0 parts 40% formaldehyde 3.7 parts of potassium bromide in parts water After mixing there were added with stirring 5.2 parts silver nitrate in 30 parts of water. The pH was adjusted to 4.

These coatings were cured at room temperature for various periods of time. The coatings were evaluated in the same way as those in Example 1. The results of the evaluation are given in Table IV.

The catalyst or reaction component may be applied in the manner described or it may be put on in conjunction with addenda such as starch or other water dispersible substances of considerable viscosity which prevent saturation of the paper or paper base stocks with the reagent and retain most of it on the surface. In cases where precoatings such as pigment binder mixtures are applied to paper before the final coating is applied, the catalyst or reaction component may be introduced as a component of the pigment binder combination applied to paper before the final coating is applied.

While this invention is intended for application in photographic practice, it may be employed to advantage in any operation in which it is desired to obtain insolubilization of hydroxylcontaining polymers under mild conditions of treatment, and where ease of control of the viscosity of the coating material is important.

The above examples describe the preferred embodiments of the invention, but other modifications are included within the scope of the invention as defined in the appended claims.

I claim:

1. A process for obtaining coatings of polyvinyl alcohol-methylol-melamine and low molecular weight condensation products thereof, comprising the steps of first coating a porous surface with citric acid and then coating with a polyvinyl alcohol-methylol-melamine mixture,

2. A method of obtaining a water insoluble coating on paper comprising coating a mixture of polyvinyl alcohol and methylol-melamine on paper to which citric acid has been applied.

3. A process for obtaining water resistant coatings of a. mixture of polyvinyl alcohol and methylol-melamine on paper, comprising first coating the paper with aluminum sulfate and then with a mixture of polyvinyl alcohol and melamine-formaldehyde.

4. A process for obtaining water resistant coatings on a paper base comprising first coating the paper base with citric acid, drying the coating and then coating with a methylol-melamine-polyvinyl alcohol composition.

5. A process for obtaining Water resistant coatings on a surface, comprising first coating the surface with citric acid, followed by coating with a methylol-melamine-polyvinyl :alcohol composition.

6. A process for obtaining water resistant coatings on a surface, comprising coating said surface with an aqueous solution of from about 2 to about 10% citric acid, drying the coating, and then applying a coating of polyvinyl alcohol-methylolmelamine mixture having a pH of 7.

'7. A process for obtaining water resistant coatings on a surface, comprising coating said surface with an aqueous solution of from 240% citric acid, drying the coating and then applying a coating of a composition containing methylo1-melamine condensate and polyvinyl alcohol.

8. A process for obtaining coatings of polyvinyl alcohol-methylol-melamine and low molecular weight condensation products thereof, comprising the steps of first coating a porous surface with an acidic catalyst selected from the group consisting of citric acid and aluminum sulfate, and then coating with a polyvinyl alcohol-methylol-melamine mixture.

Name Date Howald July 10, 1945 OTHER REFERENCES Elvanol, issued by E. I. du Pont de Nemours 8; Co., Inc. (1947), pages 34-35.

Number 

8. A PROCESS FOR OBTAINING COATINGS OF POLYVINYL ALCOHOL-METHYLOL-MELAMINE AND LOW MOLECULAR WEIGHT CONDENSATION PRODUCTS THEREOF, COMPRISING THE STEPS OF FIRST COATING A POROUS SURFACE WITH AN ACIDIC CATALYST SELECTED FROM THE GROUP CONSISTING OF CITRIC ACID AND ALUMINUM SULFATE, AND THEN COATING WITH A POLYVINYL ALCOHOL-METHYLOL-MELAMINE MIXTURE. 